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(Name,
Address and Telephone Number of Person Authorized to Receive Notices and Communications)

September 15, 2017

(Date of Event Which Requires Filing of This Statement)

If the filing person has
previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following
box. ☐

Note. Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7
for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be filed for the purpose of Section 18 of the Securities
Exchange Act of 1934, as amended (Act), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Page 4 of 5 – SEC Filing

CUSIP No. 253031108

13D/A

Page 4 of 5

This Amendment No. 1 to Schedule 13D relates to the Common Stock of Dicerna Pharmaceuticals, Inc. and amends the initial
statement on Schedule 13D filed by the Reporting Persons on April 13, 2017 (the Initial Statement and, as further amended by this Amendment No. 1, the Schedule 13D). Capitalized terms used but not defined herein
shall have the meanings ascribed to them in the Initial Statement.

Item 5.

Interest in Securities of the Issuer

Item 5 of the Initial Statement is hereby amended and restated as
follows:

The information set forth in or incorporated by reference in Items 2, 3 and 4 and on the cover pages of this Schedule 13D is incorporated by
reference in its entirety into this Item 5.

(a) (c) The percentage of the Issuers outstanding shares of Common Stock held by the
Reporting Persons is based on 20,844,429 shares of Common Stock outstanding as of August 9, 2017, as reported by the Issuer in its Quarterly Report on Form 10-Q, filed with the Securities and Exchange Commission on August 10, 2017.

On April 11, 2017, BC LS and BCIP LS purchased 226,130 and 23,870 shares of Redeemable Convertible Preferred Stock, respectively, in the Private
Placement. As further described in Item 4 of this Schedule 13D, each holder of Redeemable Convertible Preferred Stock is entitled to receive cumulative dividends on the Accrued Value of each share of Redeemable Convertible Preferred Stock and
each share of Redeemable Convertible Preferred Stock is subject to conversion, at the option of the holder, into a number of shares of Common Stock calculated by dividing (i) the sum of the Accrued Value plus an amount equal to all accrued or
declared and unpaid dividends on the Redeemable Convertible Preferred Stock that have not previously been added to the Accrued Value by (ii) the Conversion Price in effect at the time of such conversion.

As of September 15, 2017, the 226,130 shares of Redeemable Convertible Preferred Stock held by BC LS were convertible into 7,459,625 shares of Common
Stock and the 23,870 shares of Redeemable Convertible Preferred Stock held by BCIP LS were convertible into 787,100 shares of Common Stock. Accordingly, pursuant to Rule 13d-3 of the Act, BC LS may be deemed to beneficially own 7,459,625 shares of
Common Stock, which constitutes approximately 26.4% of the outstanding Common Stock of the Issuer, and BCIP LS may be deemed to beneficially own 787,100 shares of Common Stock, which constitutes approximately 3.6% of the outstanding Common Stock of
the Issuer.

(d) Except as otherwise described in this Item 5, no one other than the Reporting Persons has the right to receive, or the power to
direct the receipt of, dividends from, or the proceeds from the sale of, any of the Common Stock beneficially owned by the Reporting Persons as described in this Item 5.